Ion migration in nepheline: a dielectric spectroscopy and computer modelling study

Using a combination of dielectric spectroscopy and atomistic computer simul ation techniques, the dynamical behaviour of the loosely bound (Na+ and K+) channel ions in nepheline has been investigated. The low-frequency dielect ric properties of a natural Bancroft nepheline have been studied from room temperature to 1100 K. At each temperature, the dielectric constant, conduc tivity and dielectric loss were determined over a range of frequencies from 100 Hz to 10 MHz. At high temperatures a distinct Debye-type relaxation in the dielectric loss spectrum was observed; the activation energy for this process was determined to be 1.38 +/- 0.02 eV. Atomistic simulation techniq ues were used to elucidate the mechanism and energetics of cation migration . A mechanism involving the hopping of Na+ ions between oval sites and part ially occupied hexagonal (K+) sites, via a bottleneck consisting of a disto rted sixfold ring of (Al,Si)O-4 tetrahedra, was found to give a calculated energy barrier in very good agreement with the experimentally determined ac tivation energy. These results confirm the nature of the process responsibl e for the observed dielectric behaviour. Overall, this study demonstrates t he intrinsic, microscopic control of cation diffusion processes in rock-for ming minerals. Identifying specific energy barriers and preferred diffusion pathways is fundamental to the prediction of diffusion energetics.